1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus, wherein two image signals are respectively detected from the front and back surfaces of a stimulable phosphor sheet, on which a radiation image has been stored. This invention particularly relates to an improvement in a radiation image read-out method and apparatus, wherein the two image signals, which have respectively been detected from the front and back surfaces of the stimulable phosphor sheet, are added to each other in predetermined addition ratios, and an addition image signal is thereby obtained.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
As one of techniques for photoelectrically detecting the light emitted by a stimulable phosphor sheet, a novel radiation image read-out apparatus has been proposed in, for example, U.S. Pat. No. 4,346,295. In the proposed radiation image read-out apparatus, each of two photoelectric read-out means is located at one of the front and back surfaces of a stimulable phosphor sheet. Also, stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, are irradiated to one surface or both surfaces of the stimulable phosphor sheet. Light, which is emitted by the front surface of the stimulable phosphor sheet, and light, which is emitted by the back surface of the stimulable phosphor sheet, are respectively detected by the two photoelectric read-out means. With the proposed radiation image read-out apparatus, a single radiation image is stored on the stimulable phosphor sheet, and the light emitted by the front surface of the stimulable phosphor sheet and the light emitted by the back surface of the stimulable phosphor sheet are respectively guided by the corresponding photoelectric read-out means. Therefore, the light guiding efficiency can be kept high. Also, the signal-to-noise ratio can be kept high by adding two image signals, which have been detected from the opposite surfaces of the stimulable phosphor sheet, to each other in predetermined addition ratios.
In cases where the stimulable phosphor sheet is used in the radiation image read-out apparatus, in which the technique for detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is employed, a peripheral portion of the stimulable phosphor sheet is often curled and rises from the middle portion of the stimulable phosphor sheet due to bending caused by the passage through a bent conveyance path in the radiation image read-out apparatus, bending caused by deterioration with the passage of time, or the like. When the curled stimulable phosphor sheet is set at a position for image readout in the radiation image read-out apparatus described above, problems are encountered with the peripheral portion of the stimulable phosphor sheet, which portion rises to one side from a reference surface due to curl, in that the spacing between one surface of the peripheral portion and the corresponding photoelectric read-out means becomes narrow, and the spacing between the other surface of the peripheral portion and the corresponding photoelectric read-out means becomes wide.
As for the photoelectric read-out means located on the side of a surface of the stimulable phosphor sheet, at which the spacing between the photoelectric read-out means and the peripheral portion of the stimulable phosphor sheet has become narrow, the efficiency, with which the light emitted by the curling and rising portion of the stimulable phosphor sheet is guided by the photoelectric read-out means, becomes higher than the efficiency, with which the light emitted by the other portion of the stimulable phosphor sheet is guided by the photoelectric read-out means. Therefore, even if the intensity of the light emitted by the rising portion of the stimulable phosphor sheet and the intensity of the light emitted by the other portion of the stimulable phosphor sheet are identical with each other, the image signal value, which represents the intensity of the light emitted by the rising portion of the stimulable phosphor sheet, will become larger than the image signal value, which represents the intensity of the light emitted by the other portion of the stimulable phosphor sheet. As a result, in a visible image reproduced from the thus obtained image signal, the image density of an image area corresponding to the rising portion of the stimulable phosphor sheet becomes higher than the image density of an image area corresponding to the other portion of the stimulable phosphor sheet.
As for the photoelectric read-out means located on the side of the opposite surface of the stimulable phosphor sheet, the spacing between the photoelectric read-out means and the aforesaid peripheral portion of the stimulable phosphor sheet becomes wide. Therefore, the efficiency, with which the light emitted by the curling and rising portion of the stimulable phosphor sheet is guided by the photoelectric read-out means, becomes lower than the efficiency, with which the light emitted by the other portion of the stimulable phosphor sheet is guided by the photoelectric read-out means. Accordingly, in a visible image reproduced from the thus obtained image signal, the image density of an image area corresponding to the rising portion of the stimulable phosphor sheet becomes lower than the image density of an image area corresponding to the other portion of the stimulable phosphor sheet.
The problems described above with respect to curl of the stimulable phosphor sheet also occur due to waviness (continuation of rising and falling) of the stimulable phosphor sheet.
With the radiation image read-out apparatus for detecting two image signals from the opposite surfaces of the stimulable phosphor sheet, the two image signals having been detected are then added to each other. Therefore, it may be considered that, in the addition image signal having been obtained from the addition of the two image signals, the variations of the light guiding efficiencies of the two photoelectric read-out means due to the rising of the stimulable phosphor sheet compensate each other.
However, actually, the degree of variability, per unit rising or falling (unit curl or waviness) (in mm), of the light guiding efficiency of the photoelectric read-out means, which is located on the side of one surface of the stimulable phosphor sheet, and the degree of variability, per unit rising or falling, of the light guiding efficiency of the photoelectric read-out means, which is located on the side of the opposite surface of the stimulable phosphor sheet, do not necessarily coincide with each other. As a result, the total light guiding efficiency after being subjected to weighted addition varies in accordance with the degree of rising or falling of the stimulable phosphor sheet. Therefore, in the visible image reproduced from the addition image signal, the image density of the image area corresponding to the rising or falling portion of the stimulable phosphor sheet becomes different from the image density of the image area corresponding to the other portion of the stimulable phosphor sheet.